An aircraft may include various types of rotating electrical machines such as, for example, generators, motors, and motor/generators. Motor/generators are used as starter-generators in some aircraft, since this type of rotating electrical machine may be operated as both a motor and a generator.
An aircraft starter-generator may include three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter generator, and a main motor/generator. The PMG includes permanent magnets on its rotor. When the PMG rotor rotates, AC currents are induced in stator windings of the PMG. These AC currents are typically fed to a regulator or a control device, which in turn outputs a DC current if the starter-generator is operating as a generator. Conversely, if the starter-generator is operating as a motor, the control device supplies AC power.
If the starter-generator is operating as a generator, the DC current from the control device is supplied to stator windings of the exciter. As the exciter rotor rotates, three phases of AC current are typically induced in the exciter rotor windings. Rectifier circuits that rotate with the exciter rotor rectify this three-phase AC current, and the resulting DC currents are provided to the rotor windings of the main motor/generator. Finally, as the main motor/generator rotor rotates, three phases of AC current are typically induced in the main motor/generator stator, and this three-phase AC output can then be provided to a load.
If the starter-generator is operating as a motor, AC power from the control device is supplied to the exciter stator. This AC power induces, via a transformer effect, an electromagnetic field in the exciter armature, whether the exciter rotor is stationary or rotating. The AC currents produced by this induced field are rectified by the rectifier circuits and supplied to the main motor/generator rotor, which produces a DC field in the rotor. Variable frequency AC power is supplied from the control device to the main motor/generator stator. This AC power produces a rotating magnetic field in the main stator, which causes the main rotor to rotate and supply mechanical output power.
A starter-generator, such as the one described above, may be used to start the engines or auxiliary power unit (APU) of an aircraft when operating as a motor, and to supply electrical power to the aircraft power distribution system when operating as a generator. Thus, when operating as a motor, a starter-generator may be designed to supply mechanical output torque sufficient to start the engines. In addition, the starter-generator may be designed for optimal generator performance. For optimal generator performance, the exciter stator windings may be electrically connected in series with one another. However, with series coupled exciter stator windings, the exciter stator may need AC power with a relatively high voltage magnitude to generate sufficiently high rotational mechanical power when operating as a motor. This is because the exciter windings may exhibit a fairly low impedance to DC power, but a fairly high impedance to AC power. As a result, the AC power supply section of the starter-generator system may increase the size, weight, and cost of the overall starter-generator system.
Hence, there is a need for a starter-generator that, when operating as a motor, can generate torque that is sufficiently high to start an aircraft engine without adversely impacting the starter-generator's performance in the generating mode, and/or does not significantly impact the starter-generator size and/or weight and/or cost. The present invention addresses one or more of these needs.